There's just no reason why Joker nor zsasz are still alive. Killing them would be justifiable seeing as though neither have ANY sort of believable redemption angle to roll off of.
They don't need him as an arch villain, even that lame ass professor pyg dude they came up with did that role solidly enough.
Pretty much.
He only seems to go after low level mooks and no names. Never actually doing something about the Kingpins etc.
Recent Batman storyline also explains why Joker can't be killed - not that it even matters in a universe where Neron is waiting just around the corner to amp them up, Lazarus Pits are found around the world and death isn't permanent, even for one of the most famous deaths in comics, Jason.